Editing Galaxy (section)

=== Distinction from other nebulae ===
A few galaxies outside the Milky Way are visible on a dark night to the [[naked eye|unaided eye]], including the [[Andromeda Galaxy]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]], and the [[Triangulum Galaxy]]. In the 10th century, Persian astronomer [[Abd al-Rahman al-Sufi]] made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".<ref name="NSOG" /> In 964, he probably mentioned the Large Magellanic Cloud in his ''[[Book of Fixed Stars]]'', referring to "Al Bakr of the southern Arabs",<ref name="obspm2"/> since at a [[declination]] of about 70° south it was not visible where he lived. It was not well known to Europeans until [[Ferdinand Magellan|Magellan]]'s voyage in the 16th century.<ref name="obspm"/><ref name="obspm2"/> The Andromeda Galaxy was later independently noted by [[Simon Marius]] in 1612.<ref name="NSOG" />

In 1734, philosopher [[Emanuel Swedenborg]] in his ''Principia'' speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen. Swedenborg's views "are remarkably close to the present-day views of the cosmos."<ref name="Gordon2002"/>
In 1745, [[Pierre Louis Maupertuis]] conjectured that some [[nebula]]-like objects were collections of stars with unique properties, including a [[Relativistic jets|glow exceeding the light]] its stars produced on their own, and repeated [[Johannes Hevelius]]'s view that the bright spots were massive and flattened due to their rotation.<ref name=Kant_1755/>
In 1750, [[Thomas Wright (astronomer)|Thomas Wright]] correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.<ref name="our_galaxy"/><ref name=Dyson_1979/>
[[File:Pic iroberts1.jpg|thumb|right|Photograph of the "Great Andromeda Nebula" by [[Isaac Roberts]], 1899, later identified as the [[Andromeda Galaxy]]]]

Toward the end of the 18th century, [[Charles Messier]] compiled a [[Messier object|catalog]] containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.<ref name="our_galaxy" /> In 1845, [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] examined the nebulae catalogued by Herschel and observed the spiral structure of [[Whirlpool Galaxy|Messier object M51]], now known as the Whirlpool Galaxy.<ref>{{Cite magazine |last=Payne-Gaposchkin |first=Cecilia H. |author-link=Cecilia Payne-Gaposchkin |date=1953 |title=Why Do Galaxies Have a Spiral Form? |url=https://www.jstor.org/stable/24944338 |magazine=[[Scientific American]] |volume=189 |issue=3 |pages=89–99 |jstor=24944338 |issn=0036-8733}}</ref><ref>{{Cite journal |last=Steinicke |first=Wolfgang |date=2012 |title=The M51 mystery: Lord Rosse, Robinson, South and the discovery of spiral structure in 1845 |url=https://adsabs.harvard.edu/full/2012JAHH...15...19S |journal=[[Journal of Astronomical History and Heritage]] |volume=15 |issue=1 |pages=19–29 |doi=10.3724/SP.J.1440-2807.2012.01.03 |bibcode=2012JAHH...15...19S}}</ref>

In 1912, [[Vesto M. Slipher]] made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high [[Doppler shift]]s, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.<ref name=Slipher_1913/><ref name=Slipher_1915/>

In 1917, [[Heber Doust Curtis]] observed nova [[S Andromedae]] within the "Great [[Andromeda (constellation)|Andromeda]] Nebula", as the Andromeda Galaxy, [[Messier object]] [[Andromeda Galaxy|M31]], was then known. Searching the photographic record, he found 11 more [[nova]]e. Curtis noticed that these novae were, on average, 10 [[magnitude (astronomy)|magnitudes]] fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000&nbsp;[[parsec]]s. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.<ref>{{cite journal
 |last1=Curtis |first1=Heber D.
 |author-link1 = Heber Doust Curtis
 |date=1988
 |title=Novae in Spiral Nebulae and the Island Universe Theory
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=100 |page=6
 |bibcode=1988PASP..100....6C
 |doi=10.1086/132128
|doi-access=free
 }}</ref>

In 1920 a debate took place between [[Harlow Shapley]] and [[Heber Curtis]], the [[Great Debate (astronomy)|Great Debate]], concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.<ref>{{cite web
 |last1=Weaver
 |first1=Harold. F.
 |title=Robert Julius Trumpler
 |url=http://www.nap.edu/readingroom/books/biomems/rtrumpler.html
 |publisher=[[United States National Academy of Sciences|US National Academy of Sciences]]
 |access-date=January 5, 2007
 |archive-date=December 24, 2013
 |archive-url=https://web.archive.org/web/20131224112329/http://www.nap.edu/readingroom/books/biomems/rtrumpler.html
 |url-status=dead
 }}</ref>

In 1922, the [[Estonia]]n astronomer [[Ernst Öpik]] gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.<ref>{{cite journal
 |last=Öpik |first=Ernst
 |author-link=Ernst Öpik
 |date=1922
 |title=An estimate of the distance of the Andromeda Nebula
 |journal=[[The Astrophysical Journal]]
 |volume=55 |page=406
 |bibcode=1922ApJ....55..406O
 |doi=10.1086/142680
}}</ref> Using the new 100-inch [[Mount Wilson Observatory|Mt. Wilson]] telescope, [[Edwin Hubble]] was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some [[Cepheid variable]]s, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.<ref>{{cite journal
 |last1=Hubble |first1=Edwin P.
 |author-link1 = Edwin Hubble
 |date=1929
 |title=A spiral nebula as a stellar system, Messier 31
 |journal=[[The Astrophysical Journal]]
 |volume=69 |pages=103–158
 |bibcode=1929ApJ....69..103H
 |doi=10.1086/143167
|doi-access=free
 }}</ref> In 1926 Hubble produced a classification of [[Galaxy morphological classification|galactic morphology]] that is used to this day.<ref>{{cite journal
 | title=No. 324. Extra-galactic nebulae
 | last=Hubble | first=Edwin P.
 | author-link = Edwin Hubble
 | journal=Contributions from the Mount Wilson Observatory
 | publisher=Carnegie Institution of Washington
 | volume=324 | pages=1–49 | year=1926
 | bibcode=1926CMWCI.324....1H
}}</ref><ref>{{cite journal
 | title=Edwin Hubble, 1889–1953
 | last=Sandage | first=Allan R.
 | author-link=Allan Sandage
 | journal=[[Journal of the Royal Astronomical Society of Canada]]
 | volume=83 | issue=6 | pages=351–362 | date=1989
 | bibcode=1989JRASC..83..351S
 | url=https://apod.nasa.gov/diamond_jubilee/1996/sandage_hubble.html
 | access-date=April 21, 2024 | archive-date=February 5, 2024
 | archive-url=https://web.archive.org/web/20240205104433/https://apod.nasa.gov/diamond_jubilee/1996/sandage_hubble.html
 | url-status=live }}</ref>